The embryology of the chicken is the development of the chicken inside of the egg. Hatchery managers need to be able to differentiate between normal and abnormal embryos and identify possible causes of embryo mortality during incubation.
Fertilisation of the germinal disc by the sperm takes place in the infundibulum about 15 minutes after its holding follicle has released the yolk. Cell division to create the new embryo starts about five hours after fertilisation and continues while the egg passes along the oviduct and after the egg is laid. It is generally said that the hen’s egg takes 21 days of favourable incubation conditions for the chicken to develop and hatch. However, this development takes 22 days – one day in the oviduct and 21 days in the incubator or nest.
When the sperm cell (with half the required chromosomes) fertilises the female egg cell (with the other half of the required chromosomes) it forms the zygote, which is a single cell with the correct number of chromosomes. About five hours after fertilisation the zygote enters the isthmus and it is here that the new embryo starts to develop by simple cell division. By the time the egg leaves the isthmus, the zygote, now called the blastoderm or embryo, comprises eight cells and after four hours in the uterus it has grown to 256 cells.
Initially the dividing cells form one layer over the yolk, but as cell division continues two layers are formed. These are called the ectoderm (uppermost) and the endoderm (underneath) layers. At about this stage the central cells of the blastoderm separate from their contact with the yolk to form a cavity. It is in this cavity that subsequent embryo development occurs. Soon after the formation of the ectoderm and endoderm, a third layer of cells called the mesoderm, or middle layer, is formed.
From this stage on, the organs and tissues of the bird will develop from these three layers of cells.
Another important development at this stage is the way the cells change to allow the production of the different types of cells that make up the tissues. By the time the egg is laid the embryo consists of many cells differentiating into the various tissues, organs and body systems.
The fowl retains some vestiges of the characteristics of its reptilian ancestors. One characteristic in particular is the influence of ambient temperature during the post laying period on embryonic development. When the temperature of the egg is below 20°C, the embryo becomes dormant and most development stops. When the temperature rises above 20°C, embryonic activity re-initiates. This temperature of about 20°C when embryonic activity starts or stops is often referred to as a physiological zero.
Fluctuating temperatures above/below 20°C will create a start/stop response in embryonic development, and each succeeding response progressively weakens the embryo. The temperature must be increased to the required 37-38°C for optimum development to occur. Failure to satisfy this need leads to significantly weaker embryos. To retain maximum viability of the embryo, hatching eggs should be processed and placed in cool storage below 20°C as soon as possible after collection and held at that temperature until the pre-warming process just prior to setting the eggs in the incubator. Once in the incubator, the temperature must be controlled within very close parameters.
Because the avian embryo has no anatomical connection to the hen, all of its nutritive requirements, except oxygen, must be contained within the egg. From very early on, the embryo develops special membranes external to its body to access the nutrients in the egg and to carry out essential bodily functions.
There are four of these special membranes and their names and functions are as follows:
To better carry out an investigation into poor hatchability it is necessary to have knowledge of the way the embryo develops from day to day. This allows the hatchery manager to determine at what age/stage embryos may have died. This is important information when attempting to identify the cause of any poor results.
Photographs of the following steps may be viewed by clicking here.
The normal hatching position is:
With natural incubation the chicks hatch over a relatively short period of time. This is despite the eggs being laid in the nest over a period of several days and the hen sitting on different eggs for different periods of time. This indicates that there is some system to synchronise the hatching process. It is now known that the different embryos communicate with each other by a series of clicking sounds, the rate of clicking being the important feature. Ensuring the eggs on the hatching trays are in contact with each other facilitates the synchronisation of hatching where the eggs are incubated in a modern machine. This helps to reduce the time between when the first and last chicks hatch.